1. Field of the Invention
The invention relates to a novel process for the preparation of homogeneous or heterogeneous catalysts useful in the polymerization of olefins comprising a metallocene and the reaction product of a hydrocarbylaluminum and a hydrocarbylboroxine.
2. Background of the Invention
Traditionally, ethylene and 1-olefins have been polymerized or copolymerized in the presence of hydrocarbon insoluble catalyst systems comprising a transition metal compound and an aluminum alkyl. More recently, active homogeneous catalyst systems comprising a bis(cyclopentadienyl)titanium dichloride or a bis(cyclopentadienyl)zirconium dichloride, and an alumoxane (the reaction product of an aluminum trialkyl and water) have been found to be useful for the polymerization of ethylene.
These later-developed homogeneous metallocene alumoxane catalysts represent a significant advance in the art of olefin polymerization. Advantages over the traditional Ziegler-Natta catalysts include a higher activity obtained for ethylene polymerization. Another significant advantage is that, unlike olefin polymers produced in the presence of conventional heterogeneous Ziegler catalysts, terminal unsaturation is present in polymers produced in the presence of these homogeneous catalysts.
Teachings relating to these homogeneous metallocene alumoxane catalysts include European Patent Application No. 0036242 which discloses a process for preparing ethylene and atactic propylene polymers in the presence of a halogen-free Ziegler catalyst system of (1) a cyclopentadienyl compound of the formula (cyclopentadienyl).sub.n MeY.sub.4-n in which n is an integer from 1 to 4, Me is a transition metal, especially zirconium, and Y is either hydrogen, a C.sub.1 -C.sub.5 alkyl or metallo alkyl group or a radical having the following general formula: CH.sub.2 AlR.sub.2, CH.sub.2 CH.sub.2 AlR.sub.2 and CH.sub.2 CH(AlR.sub.2).sub.2 in which R represents a C.sub.1 -C.sub.5 alkyl or metallo alkyl group; and (2) an alumoxane. Additional teachings of homogeneous catalyst systems comprising a metallocene and alumoxane are European Patent Application No. 0069951 of Kaminsky et al., U.S. Pat. No. 4,404,344 issued Sept. 13, 1983 of Sinn et al., and U.S. application Ser. Nos. 697,308 filed Feb. 1, 1985; 501,588 filed May 27, 1983 and now U.S. Pat. No. 4,522,982; 728,111 filed Apr. 29, 1985 and 501,740 filed June 6, 1983 and now U.S. Pat. No. 4,530,914, each commonly assigned to Exxon Research and Engineering Company.
Nevertheless, these homogeneous metallocene alumoxane catalysts suffer from a disadvantage in that the ratio of alumoxane to metallocene is high, for example on the order of 1,000 to 1 or greater. Such voluminous amounts of alumoxane often requires extensive treatment of the polymer product in order to remove undesirable aluminum. A second disadvantage of the homogeneous catalyst system, which is also associated with traditional heterogeneous Ziegler catalysts, is the multiplicity of delivery systems required for introducing the individual catalyst components into the polymerization reactor.
U.S. Pat. No. 4,808,561 to Welborn, hereby incorporated by reference as if fully set forth, provides a supported metallocene alumoxane catalyst for olefin polymerization which can be usefully employed for the production of low, medium and high density polyethylenes and copolymers of ethylene with .alpha.-olefins having 3 to 18 or more carbon atoms and/or diolefins having up to 18 carbon atoms or more. The supported catalyst will polymerize olefins at commercially acceptable rates without the presence of the objectionable excess of alumoxane required in the homogeneous system. The catalyst comprises the reaction product of at least one metallocene and an alumoxane in the presence of a support material thereby providing a supported metallocene-alumoxane reaction product as the sole catalyst component.
Supported metallocene alumoxane catalysts, however, suffer a disadvantage in common with their homogeneous counterparts, namely, the method of preparation of the alumoxane component. Thus, for instance, while the '561 patent discloses that the alumoxanes can be prepared in a variety of ways, they are preferably prepared by contacting water with a solution of aluminum trialkyl, such as, for example, aluminum trimethyl, in a suitable organic solvent such as benzene or an aliphatic hydrocarbon. For example, the aluminum alkyl may be treated with water in the form of a moist solvent. In a preferred method, the aluminum alkyl, such as aluminum trimethyl, can be contacted with a hydrated salt such as hydrated copper sulfate. This method comprises treating a dilute solution of aluminum trimethyl in, for example, toluene with copper sulfate heptahydrate.
In many of these processes, because of the highly exothermic nature of the reaction between the water and the hydrocarbylaluminum, there is a high risk that the reaction can get out of control or even become explosive. While the use of CuSO.sub.4.5H.sub.2 O as source of water allows the slow addition of water, thereby reducing the risk of local excesses of water and consequently the probability of a runaway or explosive reaction, the method suffers some disadvantages. For example, the Cu(II) may be reduced to Cu(I) or even to metallic copper during the reaction with an alkylaluminum, such as trimethylaluminum. This reduction of the copper ion can lead to the introduction of undesirable types of functionalities, such as sulfate groups or even copper, into the alumoxane preparation. Prior to using the alumoxane product as a component of a catalyst system in a polymerization process, it must therefore be filtered, purified and recrystallized. Otherwise, adverse conditions may exist during the polymerization, and the quality and quantity of the polymer may be adversely affected. Another disadvantage associated with using CuSO.sub.4.5H.sub.2 O to prepare alumoxane is the low alumoxane yield, about 30% based on the aluminum trialkyl employed. Our copending application, U.S. Ser. No. 391,222, filed Aug. 8, 1989 and its parent U.S. Ser. No. 210,881, both hereby incorporated by reference as if fully set forth, however, teach an alternative, safer method of preparing alumoxanes.
In the preferred embodiment, a hydrocarbon soluble liquid trihydrocarbylboroxine, (RBO).sub.3, is first prepared. This can be accomplished by combining boron oxide with trihydrocarbylborane according to the following stoichiometry: EQU B.sub.2 O.sub.3 +R'.sub.3 B.fwdarw.(R'BO).sub.3
wherein R' can be a C.sub.1 -C.sub.10 alkyl group or a C.sub.6 -C.sub.10 aryl group, desirably a C.sub.1 -C.sub.4 alkyl group and preferably R' is methyl, ethyl, propyl, butyl or mixtures thereof. Next, trihydrocarbylboroxine is combined with trialkylaluminum to form an alumoxane and trihydrocarbylborane according to the following theorized stoichiometry: EQU (n+m)(RBO).sub.3 +3(n+m+1)R.sub.3 Al.fwdarw.3(RAlO).sub.m +3[R--(R--Al--O).sub.n --AlR.sub.2 ]+3(m+n)R.sub.3 B
wherein the sum (m+n) is 4-80, preferably 8-80, and most preferably 10-60. The R groups of the trialkylboroxine and the trialkylaluminum can be the same or different. If the R groups are different, a mixture of alumoxane containing a mixture of R groups is produced.
Since trihydrocarbylboroxine is soluble in the hydrocarbon solution, it permits a homogeneous reaction to occur with the trialkylaluminum. This results in better control of the reaction stoichiometry and product properties, e.g., the degree of oligomerization, m and n.
There yet exists a need for a simple, safe method of preparing a metallocene-alumoxane-based catalyst effective for the polymerization of olefins that avoids the need to separately produce an alumoxane by reacting a hydrocarbyl aluminum with free water, with the hazards attendant in such reacting, or reacting a hydrocarbylaluminum with a hydrated salt capable of releasing water and subsequently processing the reaction product to remove metal ions which constitute a contaminant.